1. Field of the Invention
The present invention relates to a light emitting device including a combination of a light emitting element and a phosphor for use in general lighting.
2. Description of the Background Art
Conventionally, in the field of lighting, bulbs have mainly been used. However, in recent years, with significant improvement of optical characteristics of light emitting diodes or semiconductor lasers from the ultraviolet region to the blue region, white light emitting devices that use them as excitation light sources and are combined with phosphors have become commercially available. Some white light emitting devices are used in the field of lighting and are establishing themselves in the field of solid-state lighting.
An optical structure of such a white light emitting device has been developed which includes a light emitting element in the near-ultraviolet region of 350 nm-420 nm, which is combined with a phosphor absorbing excitation light therefrom and emitting light in red, green, blue to obtain white (see for example Japanese Patent Laying-Open No. 2002-203989). However, in such a configuration, it has been pointed out that the luminous flux of the white system is reduced in relation with the color balance.
Then, for example, WO03/032407 discloses a semiconductor light emitting device emitting white-based light with high luminous flux and high general color rendering index (Ra) by combining blue, yellow, green, red phosphors with a near-ultraviolet light emitting element, in place of the light emitting device disclosed in Japanese Patent Laying-Open No. 2002-203989. Furthermore, in WO03/032407, a novel yellow phosphor (SrBaCaEu)2SiO4 activated with rare-earth elements is disclosed as a yellow phosphor, which efficiently absorbs excitation light from the near-ultraviolet light emitting element.
However, in Example 1 of WO03/032407, the theoretical efficiency limit is only about 140[lm/W] and the general color rendering index (Ra) is only about 68. On the other hand, Example 3 of WO03/032407 which has a changed combination of phosphors achieves the theoretical efficiency limit of about 170[lm/W] and the general color rendering index (Ra) of about 92.
In this manner, when general phosphors activated with rare-earth elements are used, it is difficult to obtain the optimum combination of a desired emission wavelength by rare-earth elements serving as the luminescent center and a base material having an absorption band for absorbing excitation light enough, and it is difficult to select such an emission peak wavelength or an emission spectrum that optimizes the theoretical efficiency limit. Therefore, generally, in reality, various combinations of phosphors are tried to select the optimum combination of the theoretical efficiency limit and the general color rendering index (Ra). In the conventional structure including a yellow phosphor in addition to blue, green, red phosphors, the white luminous flux is improved but the theoretical efficiency limit has not been optimized enough.
Here, the theoretical efficiency limit refers to a luminous flux obtained when the internal quantum efficiency (defined as the ratio of photons of fluorescence/photons of excitation light) of a phosphor is 100% and all the excitation light is absorbed in the phosphor, where the phosphor is irradiated with excitation light of light intensity 1 W. For example, in Example 1 of WO03/032407, the luminous flux of about 140(lm) is obtained in the case as noted above. It is noted that the luminous efficiency (lm/W) in the actual light emitting device is obtained by multiplying the theoretical efficiency limit of the phosphor by the internal quantum efficiency of the phosphor, the absorptance of excitation light and the luminous efficiency of the light emitting element. Therefore, in order to improve the luminous efficiency of the light emitting device, the theoretical efficiency limit of the phosphor is preferably increased as much as possible.
In addition, the so-called general color rendering index (Ra) is preferably high which is an index in the use of a light emitting device as lighting to indicate a difference in vision of a color between an object viewed under sunlight (reference light D65) and an object viewed using a light source of a light emitting device. This general color rendering index (Ra) is generally 80-90 or higher for use in the general lighting, for example, such as fluorescent lamps. A value in this range is also requested in a semiconductor light emitting device using a light emitting device as a solid-state light source. Then, this general color rendering index (Ra) is also heavily dependent on the shape of emission spectrum.
Furthermore, a light emitting device having better characteristics may be obtained when the special color rendering index (R9) is higher which indicates how clean the red color is, when a red object is viewed. However, the documents that disclose prior arts do not describe those points.